The long-term objective of this proposal is to determine the mechanism of autoreactive B cells in systemic lupus erythematosus. The present interest in anti-B cell therapy for systemic lupus erythematosus (SLE) and other diseases underscores its importance. The overarching hypothesis is that anti-self B cells of different subsets in SLE are activated by different mechanisms and pre-programmed to have unique roles in pathogenic anti-self responses. We will focus our efforts on the mechanism of activation of B cell specific for the Smith (Sm) antigen, a ribonucleoprotein commonly targeted in SLE. Our previous work indicates that anti-Sm B cells of the follicular (FO) B cell subset are non-functional, but, paradoxically, also indicates that some anti-Sm B cells are selected into the marginal zone (MZ), pre-plasma, and B-1 cell subsets, and are functional. The distinct phenotypes, activation states, and anatomical locations of B cells of each subset are designed to provide a layered and interdependent, protective response to foreign antigens. However, we know little about the interplay between the B cells of these subsets in responses to self-antigen, but preliminary data indicates that they make unique contributions to the anti-Sm response in autoimmunity with implications for pathogenesis. We also find that dendritic cells (DCs) activate anti-Sm MZ B cells, but normally this ability is carefully regulated to prevent concurrent T cell activation. However, in autoimmunity, a deficiency in the activation receptor Fas, allows anti-Sm B cell activation concurrently with T cells, with the potential to generate pathogenic antibodies. We propose three aims to determine the role of B cells of each subset to autoimmunity and the mechanism of anti-Sm B cell activation by DCs. In the first aim, we will determine the contributions and interdependence of B cells of each subset the anti-Sm responses in autoimmune mice. In the second aim, we propose to determine the mechanism of anti-Sm B cell activation by normal and Fas-deficient DCs, and in the final aim, we will determine how Fas regulates the ability of DCs to activate anti-Sm B cells. The proposed experiments will aid in the identification of new therapeutic targets that affect pathogenic mechanisms for B cell activation. PUBLIC HEALTH RELEVANCE Antibodies specific for self-proteins cause many autoimmune diseases, including systemic lupus erythematosus. How B-lymphocytes that produce these pathogenic anti-self antibodies are activated is largely unknown. In this proposal, we will determine how these B-lymphocytes are activated for the purpose of identifying mechanisms that can be therapeutically targeted in patients with these diseases.